This application relates to electric lamps that have a light source which produces ultraviolet radiation, and to improvements in such lamps for reducing photoelectron production caused by the ultraviolet radiation within the lamp.
The cause of photoelectron production in electric lamps, and the consequent problems are well documented. See, for example, Waymouth, Electric Discharge Lamps (MIT Press, 1971), Section 10.5. As Waymouth describes in detail, some electric lamps, particularly electric discharge lamps containing ionized mercury, emit a strong flux of ultraviolet radiation. These lamps typically are comprised of a discharge vessel in which an arc discharge occurs, mounted within an outer envelope by metal support structure. The ultraviolet radiation from the discharge vessel strikes the metal support causing the emission of photoelectrons.
Photoelectron emission can be very detrimental to certain electric lamps. In metal halide discharge lamps the discharge vessel is typically quartz and contains during lamp operation an ionized plasma of mercury, sodium, a halogen such as iodine, and other metals such as scandium and thallium and various compounds of these elements. Sodium ions have a high rate of diffusion through heated quartz. Photoelectrons which collect on the outer surface of the discharge vessel create a negative potential that attracts the positive sodium ions and accelerates their diffusion through the wall of the discharge vessel. The production of photoelectrons substantially accelerates the depletion of sodium within the discharge vessel and thus shortens the useful life of the lamp.
Different measures have been taken in order to diminish the effect of the photoelectrons. One is to reduce the production of photoelectrons by covering metallic structure within the lamp outer envelope with an appropriate material. In some lamps the light source, such as the discharge vessel of a discharge lamp, is supported by a metallic frame structure having a supporting member, typically a rod, extending along the length of the light source. The support rod is metal and is exposed to ultraviolet radiation and consequently emits a substantial flux of photoelectrons.
U.S. Pat. No. 3,484,637 (van Boort et al) discloses a mercury vapor discharge lamp in which the supporting metal rod of the discharge tube support frame is covered by a ceramic tube comprised of alumina and silica. The ceramic tube shields the metal rod from ultraviolet radiation and reduces the production of photoelectrons.
A similar approach is disclosed in U.S. Pat. No. 3,780,331 (Knochel et al) which discloses a discharge lamp in which a ceramic or fused quartz tube covers a support conductor which supports the lamp discharge tube. This patent also teaches the addition of a photoelectron collector and the use of a stainless steel support conductor with a chrome oxide surface, in substitution for the nickel plated iron support conductor usually used. U.S. Pat. No. 4,171,498 (Fromm et al) likewise teaches the use of a quartz tube surrounding the support conductor for reducing photoelectron emission.
All of the lamps disclosed in the above-mentioned references include a straight support rod. The rod is straight because the ceramic or glass tube covering the rod must be straight. It would be impracticable to fabricate curved ceramic or glass tubes so as to allow the use of curved support rods.
An altogether different approach to reducing photoelectron emission is to eliminate the support rod extending along side the discharge tube. U.S. Pat. No. 3,424,935 (Gungle et al) discloses a metal halide lamp having metallic structure for supporting respective ends of the discharge tube at the opposite ends of the lamp outer envelope. No metal support rod extends along the length of the discharge tube for providing mechanical support, but a fine tungsten wire provides a conductive path between the lamp base and the far end of the discharge tube. The elimination of the metal support rod eliminates the source of a substantial portion of the photoelectrons produced by the ultraviolet radiation emitted from the discharge tube.
Another measure for reducing photoelectron production, applicable to both lamp types just mentioned, is the introduction of a gas, such as nitrogen, into the outer envelope. The nitrogen reduces the number of photoelectrons that reach the discharge tube and thus collect on it and impart a negative potential to the tube outer wall.
Still another technique is to construct the discharge tube support so that its metal elements are as far from the discharge tube as is possible. This appears to be of limited effectiveness; however, without other measures being taken. Waymouth reports that the photoelectric current in a lamp having an evacuated outer envelope and a metallic support rod three inches away from the discharge tube is greater than in a lamp having a nitrogen atmosphere in the outer envelope and the metallic support rod only one-half inch away from the discharge tube.
Ideally, one would construct a lamp using more than one photoelectron reduction technique in the lamp. Typically, metal halide lamps are made with a nitrogen atmosphere in the lamp outer envelope, and with a tubular cover, such as quartz or alumina, over the support rod of the discharge tube support structure. Because such tubular glass or ceramic covers are only practicable if made straight, their use constrains the support rod shape to straight. The only other alternative, then, is to dispense with the support rod altogether and to use structure embodying the concept disclosed in U.S. Pat. No. 3,424,935, mentioned above, or to find a new material for covering the metal structure within the lamp.
It is therefore an object of the invention to use a new material for suppressing photoelectron production in lamps having internal metallic structure.
It is another object of the invention to provide means for preventing photoelectron production from metal components within a lamp irrespective of the shape of the components.
Another object of the invention is to provide a discharge lamp having a metallic discharge tube support structure of arbitrary shape covered by a material effective to suppress the emission of photoelectrons.